Study of Low-Frequency Molecular Motions in HF, KHF2, KH2F3, and NaH2F3

Abstract

The spectra of KHF2, KH2F3, NaH2F3, and of liquid and solid HF were measured between 1200 and 30 cm—1 by the inelastic scattering of ``cold neutrons'' (0.005 eV). The spectrum of KHF2 is in agreement with previous infrared measurements and shows a peak at 147 meV corresponding to a deformation frequency ν2 of the F–H···F group and a broad peak at 13 meV due to a libration of this group. The observed spectra for KH2F3, NaH2F3, and solid and liquid HF were compared with that for KHF2 considering the known results than the F–F distance increases, the position of the H atom between the two fluorines becomes more asymmetrical, and the strength of the hydrogen bond decreases in going from the HF2 — ion to H2F3 — ion to HF. In this comparison, the known structure of the H2F3 — ion is considered as being intermediate between that of HF2 — and the zigzag hydrogen bonded chains in solid HF. As a result, the lines observed at 112 and 67 meV in the H2F3 —spectra and in HF, respectively, are associated with the deformation frequency ν2 of the H–F–H group in these compounds. In addition, the spectra of H2F3 — ions show a peak at a 15 meV attributed to a libration of the group. Both H2F3 — and HF show peaks at 8 and 7 meV, respectively, attributed to a rotational motion of one F–H···F unit around the F–F bond. The observed spectrum of HF liquid, showing a high degree of association, is also discussed with respect to these motions.